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In the widebander genre, designers keen on hitting the full in range (in the bassment rather than treble) routinely add very sizeable rear horns. Having lived with Voxativ's Ampeggio which does hit an astonishing 40Hz from its lone driver, the Wave 40 + Submission play in a different league.

Even if the majority gains come from the unfair advantage of Zu's tower, I have to believe that had soundkaos pursued lowering their own F3 with greater internal air volume and a longer line, they would also have compromised their exceptional transparency and gushing fleetness.

I'll put this differently to solidify my point.
Comparing Submission gains to any prior passive speaker hosted or owned including Avantgarde Acoustic's semi-active Duo—the single Zu is far superior—makes me seriously question the wisdom (or insanity) of pursuing low bass with standard boxes. Harnessing a purpose-built bass machine for the most demanding lowest octave not only makes more theoretical sense, it's completely justified by experience. That not all households could justify Submission's dimensions goes without saying. A half-sized $2.500 Undertone with the same electronics and woofer but simpler construction would clearly be more decor friendly and likely accomplish 90%.

Here one must mention the usefulness of Zu's 6dB LF boost. It affects a narrow half-octave width which may be set anywhere between 20 and 50Hz. I selected 26Hz to cover 20-30Hz. This created proper in-room power response without muddying up what occurs higher up. That's key. Little in hifi is quite as annoying as lumpy boomy bass. At this juncture my Submission exploits with the Wave 40 and Druid V really had me at hallo. It took 11 years of reviewing to finally experience a superior sub not as the usual monitor mate but as a yeoman buddy for speakers which most would consider sufficiently full-range on their own.

Some might smack their forehead with a 'duh dude!' response. But I predict that many otherwise experienced listeners haven't tried this before either. In the end however this report might be primarily for those who are still contemplating their 'last' speaker. I'd especially encourage them to contemplate something smaller and cheaper without giving up on their full-range ambitions - in fact having those cashed in at a far more mature level than envisioned. Getting a proper dealer demo of such a setup will predictably clash with deep-seated audiophile snobbery that subwoofers are only for home theater and lifestyle folks who must have those 'invisible' micro speakers. In fact a reader with Druid V curious about Submission had his dealer tell him point blank not to ever expect a floor sample to try. He'd order one prepaid but had no interest to stock one for demo purposes nor faith that this combination stood any chance at properly meshing.

Someone other than Zu already there is Joe Jouhal of JoSound with his bamboo-clad JO20 stereo subwoofer console with isolated top as equipment support which augments the Voxativ-fitted JO30 and smaller JO45 Jordan widebander models.
On this particular path, the hunt is for a sonically superior main speaker that'll do 40-50Hz, then adding a purpose-built true subwoofer with the right adjustment features for the remainder. It simply means thinking about subs and the importance of infrasonics done right in a different way.

To conclude the soundkaos chapter,
after designer Martin Gateley heard the Submission with his speakers, he asked for Sean Casey's email. Booked to attend HighEnd Munich 2013 with Bakoon, he wanted to inquire whether a Submission loaner might be available for his exhibit. As a refined listener who went to costly extremes with the build of his tonewood enclosures, it's fair to say that Martin didn't consider this sub addition to his widebander barbaric, redundant, ill-advised or patchy. Au contraire. It completed the picture in ways neither of us quite expected.

Interlude. During the Birth 100 review for Black Pearls Audio, I moved my big AudioSolutions Rhapsody 200 from Lithuania in to better complement the amp's target audience and price. This 5-driver 3-way vents its paralleled woofers with two sizable slightly underdamped ports. Impedance thus swings from 2.8Ω at 9kHz to 20Ω at 17Hz. Until I acquire a pair of Bruno Putzeys' Mola-Mola Ncore 1200 monos—I had previewed their stupendous control over this load with the Acoustic Imagery Atsah—I have those ports plugged. Even so the shift from the Bakoon/Wave to the Birth/Rhapsody was dramatic in how illumination and sparkling refinement north of the upper midrange shut down.

Because my writing desk sits diagonally in the listening room's closer rear corner to the left (in the photo above right behind the two left chairs and adjacent to the two listening chairs), I routinely listen while I write, mostly at levels low enough to hear myself think. This nonetheless has me very aware of any corner pressurization boom generated behind the left speaker which I essentially face head-on.
Even if such high-pressure corner nodes don't—seem to—telegraph directly as boom in the central listening seat, they most assuredly are audible as a suffocating pillow on the gossamer-fine upper harmonics exactly as though one overdamped a room with too much absorptive panels. Whilst such devices do clean up bass, they can also kill off the light, air and effervescence I think of as the overtone-generated spark of life. I still have four professional room absorbers in the garage—two corner traps, two first-reflection sidewall absorbers—which I quickly removed after first moving here for exactly that murderous offense.

The upshot of this observation is very simple. In a standard short-wall orientation room which by necessity will place your speakers within one or two meters of the front corners, the more bass-extended those speakers get, the more they will pressurize and load those corners with LF energy if they're ported. That corner turbulence now becomes an upper-harmonic damper.
A better solution is to replace those bass-extended speakers with smaller ones and relocate the actual bass generator into the middle of the front wall. Now low frequencies energize the room differently without building up the same intense pressure in the front corners. Just because you're not sitting far off-center to hear this corner compression directly as I do on my work desk shouldn't imply that it has no influence on your sweet spot!

This gets us at my ?/= equation. Would you then fare better with a smaller speaker and single sub? I can't speak of long-wall setups. I've never had one for my primary listening space. For short-wall setups with room widths of ca. 5m or less, I predict that the question mark in nearly all cases becomes a solid yes. Now the trade-off becomes between raw air movement (the bigger speakers displace more air in the bass) versus cleaner more articulate bass that goes lower with far superior control plus better HF performance and less in-room bass energy swamping the critical vocal range. I don't know about you. I only know what I'd much rather have. In the context of the above visual, the speakers surrounding the Submission sub are arguably still unnecessarily 'big'. An even simpler 6" - 7" two-way equivalent with single mid/woofer would probably be cheaper and with the sub addition perform just as well.